1. Field of the Invention
The present invention relates to a paper feed device and an image forming apparatus having the paper feed device.
2. Discussion of the Background Art
FIG. 1 is schematic view illustrating a background paper feed device 20. The paper feed device 20 may be used, for example, in image forming apparatuses such as a photocopier, a facsimile machine, a printer, etc. Referring to the FIG. 1, the paper feed device 20 includes a paper feed roller 21 on a shaft 22, a paper separating device 25 having a friction pad 25F, and a paper tray 27 having a tray friction pad 23. The paper tray 27 loads a plurality of sheets of paper S. The paper feed roller 21 rotates in a clockwise direction so that a top sheet of papers S is fed toward the paper separating device 25. When plural sheets of papers are fed into the paper separating device 25 one at a time, the paper separating device 25 separates the top sheet from the other sheets and further transports the top sheet. The tray friction pad 23 prevents plural sheets of paper from being fed out at the same time, especially when the paper tray 27 loads a relatively small number of sheets, for example, such as twenty sheets or less.
In general, the outer surface of the paper feed roller 21 has substantially a cylindrical shape. The outer surface, however, may be worn by friction between the outer surface and the sheets of paper being fed out over a long period. In addition, the wear may not occur uniformly, and thereby the outer surface of the paper feed roller 21 may acquire a non-cylindrical shape. When the paper feed roller 21 has worn non-uniformly to acquire, for example, a cone shape, the pressure at a contacting point between the outer surface and a sheet of paper being fed out varies depending upon an axial position of the paper feed roller 21.
FIG. 2 is an explanatory graph illustrating a relationship between axial positions of a worn paper feed roller 21 and pressures against sheets of paper. In FIG. 2, the paper feed roller 21 has been worn into a barrel shape. As illustrated, the pressure at a contacting point between the outer surface and a sheet of paper being fed out varies depending upon the axial position of the paper feed roller 21 or depending upon a diameter of the outer surface of the paper feed roller 21 at that axial position. At a position where the diameter is large, the pressure has a large value denoted as P1, and at a position where the diameter is small, the pressure has a small value denoted as P2. Such a pressure difference P1-P2 may cause a malfunction in a paper feeding operation, such as a paper skew or a paper jam. The diameter of the outer surface of the paper feed roller 21 may also vary along an axial position of the paper feed roller 21 due to errors arising during the manufacture of the roller, and such errors may cause similar malfunctions.
When the paper feed roller 21 is damaged or worn out, the damaged or worn paper feed roller must be replaced by a new one. Generally, a worn paper feed roller is detached by pulling it out and a new roller is attached along a direction of the axis of the shaft 22. Therefore, a replacing operation of the paper feed roller requires a relatively long time, and a down time of an image forming apparatus using the paper feed device becomes relatively long.
The present invention has been made in view of the above-discussed and other problems and is intended to overcome the above-discussed and other problems associated with the background methods and apparatus. Accordingly, one object of the present invention is to provide a novel paper feed device and an image forming apparatus having the paper feed device that can perform a quality paper feeding operation for a relatively long period.
Another object of the present invention is to provide a novel paper feed device and an image forming apparatus having the paper feed device for which a maintenance operation can be performed in a relatively short time.
To achieve these and other objects, the present invention provides a novel paper feed device and an image forming apparatus having the paper feed device which includes paper feed rollers to feed a sheet of paper. A paper loading device loads at least a sheet of paper and lifts the loaded sheet of paper toward the paper feed rollers. A pressure differences decreasing device is mounted on the paper loading device and decreases differences of pressures among a plurality of positions where the paper feed rollers contact the sheet of paper.